Cholesteric liquid-crystal polymers are known, for example, from DE-A-42 40 743, EP-A-0 724 005 and DE-A-195 38 700.
They possess a twisted structure with a pitch that corresponds to a wavelength of light in the range from UV to IR. Cholesteric liquid crystals, or generally liquid-crystalline substances with a chiral phase, which possess a twisted structure with a desired pitch, can be obtained, for example, from nematic liquid-crystal polymers (LCPs) by doping them with a chiral substance or incorporating the chiral substance into the polymer. The nature and proportion of the chiral substance determine the pitch of the twisted structure and thus the wavelength of the reflected light. The twist of the structure can be either left- or right-handed. Moreover, the starting substances possess groups which are amenable to addition polymerization, polycondensation or polyaddition.
Depending on the content of chiral monomer, the material displays pronounced color effects which are based on selective reflection at the helical superstructure. The exact reflection color here depends on the viewing angle and, in particular, on the pitch of the helix. For a given viewing angle--for example, perpendicular viewing of a sample--the reflection color which appears is a color with a wavelength which corresponds to the pitch of the helical superstructure. So the smaller the pitch of the helix, the shorter the wavelength of the reflected light.
In the majority of cases, cLPCs comprise aromatic molecular units. These are hampered by the disadvantage of deficient weather stability as a result, for example, of strong UV absorption.
It is known that polymerized compounds can be improved in their UV stability by the admixture of UV stabilizers. Whereas this concept is established and functions well for amorphous and partially crystalline systems, it has not to date been applicable to liquid-crystalline substances, since the liquid-crystalline phases are, as a general rule, immiscible with non-liquid-crystalline components. A UV stabilizer mixed into a liquid-crystalline substance is exuded over the course of time; in other words, there is separation, and the UV stabilizer is no longer able to protect the liquid-crystalline substance.